Tubular prostheses, commonly referred to as stents, are used in a variety of medical procedures. For example, stents are often used in connection with assisting drainage from the kidney through the ureter, from the liver through the biliary ducts, from the dorsal or ventral pancreas through the pancreatic ducts, from the gall bladder through the cystic, hepatic, or common bile ducts, and the like. A leading reason for stent deployment in ducts is to provide drainage to circumvent a blockage. Blockage of ducts in the body can be a serious and very painful affliction that can result in death if not promptly and effectively treated. Blockages can occur for a number of reasons. For example, stones or debris from such stones can pass from the kidney into the ureter, where they become entrapped. Similarly, stones or debris can pass from the gall bladder into the bile ducts, where they become entrapped. Alternatively, cysts or tumors growing against the outer wall of the ducts can cause constriction of the ducts. Similarly, internal or duct wall cysts or tumors can act to block ducts.
The main function of ureteral stents, for example, is to bypass ureteral obstruction and to provide urinary drainage from the kidney to the bladder for a period of time, typically a few days to several months. The ureteral stent is usually provided with drainage means such as a lumen for directing fluid from the renal pelvis to the bladder. Conventional stents include openings provided along the stent for communication with the lumen to aide in drainage.
Early ureteral stents were straight. As a result, after placement into the ureter, these straight stents often migrated or were expelled from the ureter as a result of peristaltic action by the ureter. Later ureteral stents, therefore, were usually designed with means of retention on one or both ends of the stent. The retention means is intended to inhibit stent migration either upward into the kidney or downward into the bladder. Retention means that have been employed are in the form of hooks, pigtails, coils, corkscrews, malecots, barbs, mushrooms, or any other practical shape that will serve the purpose.
In addition to varying lengths, ureteral stents are also made with varying diameters, e.g., from 3 French (1 mm) to 16 French (5.28 mm), and typically, 4.5 French (1.5 mm) to 8.5 French (2.8 mm), and varying degrees of hardness. Ureteral stents with smaller diameters are usually easier to insert but may provide insufficient drainage, whereas stents with larger diameters allow for increasing drainage capacity through the ureter but may be difficult to insert.
Current urinary stents comprise a shaft commonly made of either single or dual durometer polymer material. Current shaft designs often have unique profile cross-sections and hydrophilic or anti-microbial coatings, for example. This shaft typically resides in the ureter to provide drainage of urine after ureteroscopy procedures. Stiff ureteral stents are easier to insert than are softer stents, but once inserted can lead to increased patient discomfort. Stiff stents are less likely to accommodate the dynamic urinary tract anatomy that stretches and relaxes. The relative push and pull of the kidney and bladder is not accommodated for, therefore increasing patient discomfort from contact irritation of the stent within the anatomy. In addition, the lack of accommodation of the normal body movements increases the potential for the stent to migrate or dislodge from its intended location.
Anecdotally, it is believed that the softer the material, the less irritation to the ureter, and the greater the patient comfort. The lack of stiffness makes it difficult to push the stent in place, however. Hence for placement, a certain axial stiffness is built in which equates to a high level of radial stiffness. Stiff stents are believed to be felt by the muscle spasm of the ureter potentially causing patient discomfort. Further, the axial stiffness of current stents may not be ideal for comfort to the urinary tract anatomy without being felt by the patient.
Presently, most available stents are either made of silicone or of a harder polymer. Silicone may increase patient comfort, but because of the softness of silicone, it is more difficult to guide the stent into the ureter. Once in the ureter, the softness of the silicone increases the likelihood of migration of the stent because rigid retention means are not available.
Thus, although stents have been designed to address one or more of the above problems specifically, there are currently no devices incorporating features that can be used to bypass most of the aforementioned disadvantages. It would thus be desirable to have a stent that provides one or more of the following attributes, easy insertion or implantation, strong retention, and increase patient comfort.
Urinary stent typically have a bladder curl made from a soft material to prevent migration and irritation. Patient discomfort from these stents come from a number of areas. One such area is that the bladder curl can rub and irritate the inside of the bladder, especially the sensitive trigone area. Another such area is that flank pain is perceived by the patient due to the reflux occurring as the bladder voids and urine is pushed up through the stent lumen back into the kidney.